By Deepa Venkatraghvan NEW YORK ( MainStreet)--Your credit card is just like the Tree of Knowledge from the Garden of Eden. You can see the unsparing credit line extended to you but really, you cannot use it all. Well, let me put it this way: you can use all of your credit, but if you do, your credit score will take a massive hit. Deceptive, isn't it? If you've been extended a $1,000 credit line, you would assume that you're fine if you stayed within that limit and paid in full each time. The reality is quite different, because there's something called debt-to-credit limit ratio.